The role of calcium in alleviating aluminium toxicity

Effects of varying calcium (Ca) concentration at constant or varying solution ionic strength on root elongation of soybean (Glycine max (L.) Merr.) and subterranean clover (Trifolium subterraneum L.) were determined in aluminium (Al) free nutrient solution or solutions containing monomeric Al activities (σ α) of 8-10 µM for subterranean clover and 20-22 µM for soybean. In Al-free solutions, the root length for subterranean clover was not significantly influenced by Ca or ionic strength. However, soybean root length was greater at 500 µM Ca than at higher Ca concentrations. Raising the ionic strength at 500 and 5000 µM Ca significantly decreased root length. In the presence of Al, maximum root length of both species occurred at 15 000 µM Ca. Soybean root length at 500 and 5000 µM Ca was 35% and 87% respectively of that at 15000 µM Ca. The corresponding values for subterranean clover were 53% and 81%. The positive effect of Ca concentration on root length, despite a nearly constant σ α, confirms the existence of a protective action of Ca against Al toxicity. Raising the solution ionic strength at 500 µM Ca in the presence of Al improved the root growth of soybean by 86% and that of subterranean clover by 45%. At 5000 µM Ca, a small beneficial effect of increased ionic strength (14%) was found only in subterranean clover. Increasing Ca concentration in solution decreased water extractable and 0-1 M HNO extractable Al in roots of both plant species. Transfer of soybean seedlings to Al-free nutrient solutions containing 500, 1500 or 5000 pM Ca after 24, 48 or 96 h in a solution containing σ α mono of 22 µM resulted in a substantial recovery in primary root growth. Relative root lengths were in each case significantly higher at 5000 µM Ca than at 1500 or 500 µM Ca. Roots transferred to 500 µM Ca after exposure to Al for 6 or 18 h underwent a period of accelerated elongation after a lag period of 30-40 h. By 138 h there were no significant differences in root length between the unstressed control plants and those subjected to 6 or 18 h Al-stress